1. Field of the Invention
The present invention relates generally to a non-destructive detection and three dimensional sizing of subsurface flaws within a solid material and has particular applicability, but is not limited to, a detection of flaws within a metal weld.
2. Description of the Prior Art
There are two general non-destructive methods in the prior art for detecting subsurface flaws in a solid material, these being X-ray radiography detection and ultrasonic detection. The present invention relates to ultrasonic detection.
One prior art method of ultrasonic detection of flaws within a solid material is a single probe, "pulse echo", method.
Basically, the single probe pulse echo operates by emitting, from a transmit/receive probe, an ultrasound pulse and then monitoring the received echo for any pulses received which are above a set threshold value. The echo pulses are the reflections from the discontinuities surrounding any subsurface flaws. Based on the time delay of recovery of the flaw echo, the depth to the first major discontinuity of the flaw can be calculated. By scanning the probe along the test surface direction, the length of the flaw is indicated by the relative positions along the scan where its echo first appears and where the echo vanishes. However, a substantial shortcoming of the single probe pulse echo method is that it cannot accurately determine the height of the flaw. The reason, as best theorized by the present inventors is as follows: When the incident sound pulse travels downward into the test material and strikes a discontinuity at the upper portion of a first flaw there is a reflection back from that discontinuity. The single probe pulse echo method, by its usual operation, measures the time of receiving this reflection and, based on the known height of the probe above the surface and the speed of sound through the material, calculates the depth of the discontinuity, and hence the upper portion of that first flaw. Since there is a discontinuity at the lower portion of the same flaw then, in theory, if the single probe method could detect a reflection from that portion then the height of the flaw could be determined. However, this is not practicable. The main reason is that the upward reflection from that lower portion will itself be perturbed by the discontinuity at the upper portion of the flaw. In other words, the upper portion of the flaw interferes with echoes from the lower portions of the same flaw. Furthermore, because of the longer path length through the material to the lower discontinuity, that lower reflection will be of a significantly lower amplitude than the upper reflection. Since the system must detect flaws of varying depth, but not be overwhelmed by extraneous noise, the detection threshold cannot be set low enough to detect these signals. For a similar reason, when there are multiple flaws, one overlaying another, the upper flaw interferes with detection of echoes from the lower flaw. Therefore the lower flaw is not detected. Also, since the amplitude of the reflection echo is dependent on the angular orientation of the flaw, the single probe pulse echo method is prone to miss the detection of unfavorably oriented flaws, particularly those of smaller size.
There are two types of two probe methods shown in the prior art. The first is referred to as the transmission method. This method utilizes two transducers with the material under inspection being inserted between the two. An ultrasonic energy is launched from one of the transducers, propagates through the material and the intensities of the received pulses are measured by the receiving transducer on the other side. As long as a signal is received the material is considered to be defect free. A defect, on the other hand, reflects a significant portion of the propagating pulse and caused a loss of signal at the receiving transducer. This method, of course, is limited to situations where both sides of the tested material are accessible. Also it cannot readily measure defect depth. The other method, which is discussed in an article by M. G. Silk et al., "Ultrasonic Time-Domain Measurements of the Depth of Crack Like Defects in Ferritic and Austenitic Steels", Proceedings of Specialist Meeting on Ultrasonic Inspection of Reactor Components, pp. 1-17, September 1976, is termed the Time-of-Flight Diffraction Method. In this method, two transducers are arranged on the same side of a material, separated by a short distance. According to Silk, sound insonifies the material being inspected and at the same time produces what is termed as a "lateral wave" just below the surface of the material. The lateral wave provides a timing reference for subsequent signals diffracted from subsurface defects. However, no physical mechanism can be found for generating a surface wave in a solid medium from a liquid medium, especially with the incident wave in the liquid medium being nearly normal to the surface of the solid, as taught by Silk. The lack of a surface wave, in conjunction with other sonic effects inherent with the transducer arrangement taught by Silk, as well as the sonic model used for the subsequent calculations, result in that method having limited depth measurement accuracy and fault detection performance.